The invention relates to a driver circuit for actuating an electrical device via a control line and for diagnosing the state of the control line and/or of the actuated device
In modem, electronically controlled internal combustion engines, the ignition output stage is known to be actuated by the electronic engine controller using line drivers, with the line drivers needing to meet various requirements.
By way of example, the line drivers need to produce rapid switching edges having a switching time of just 0.5-2 μs even if a capacitor is grounded at the output of the line driver for the purpose of EMC suppression.
In addition, the line drivers need to be sufficiently resistant to ground and battery shorting and to any interruption in the control line.
Finally, the line drivers need to allow diagnosis of shorting to ground or to battery voltage and of any line interruption.
To achieve this diagnostic object, line drivers are known which measure the output current and compare it with the normal load current in order to be able to identify a fault in the event of any discrepancy. Thus, ground shorting results in a rise in current beyond the normal load current, while battery shorting and line interruption result in a reduction in the output current.
However, a problem in this regard is that the output current from the line driver during the turn-on phase is initially determined by the relatively large charging current for the EMC capacitor, which means that it is not possible to detect fault-related discrepancies in the much smaller load current. Thus, the charging current for the EMC capacitor can be 40 mA for example, while the normal load current is just 20 μA.
In the known line drivers, the diagnostic function is therefore not available until the EMC capacitor has been largely charged and the charging current has decayed accordingly, so that fault-related discrepancies in the load current can be detected.
A drawback of the known line drivers described above is thus that dynamic identification of a line interruption is not possible during the turn-on phase.
In addition, line drivers are known which impress a small output current of, by way of example, 10 μA on the control line during the turn-on phase, with the voltage rise at the output of the line driver being measured. With a fault-free control line, only a portion of this output current charges the EMC capacitor, while the other portion flows via the load and thereby delays the build-up of voltage on the EMC capacitor. By contrast, in the event of a fault-related line interruption, the total output current contributes to the build-up of voltage on the EMC capacitor, so that the voltage rise is faster in the event of a line interruption. Measuring the voltage rise during the turn-on phase therefore allows a line interruption to be identified for this line driver type.
However, a drawback of this is that a line interruption can no longer be identified in this manner during normal operation after the turn-on phase, since a constant current cannot be impressed during normal operation.
U.S. Pat. No. 5,043,655 discloses a circuit tester which uses two current mirror circuits to measure the current flowing out of the test piece or into the test piece, the measured value being compared with a prescribed reference value in order to identify any faults in the test piece.
In addition, DE 199 00 978 A1 discloses a circuit arrangement for ascertaining a resistance for an ignition element for an airbag in order to be able to identify a fault in the ignition element. To this end, a small test current is supplied to the ignition element and is evaluated using two current mirror circuits. In this context, the ignition element is activated by a separate occupant protection means control arrangement, however.
Also, DE 197 29 904 A1 discloses a circuit arrangement for monitoring currents flowing through a load, which likewise has a current mirror circuit. This circuit arrangement can actuate the load with high levels only, however.
Finally, HABIGER: “Handbuch Elektromagnetische Verträglichkeit” [Electromagnetic Compatibility Manual], 2nd edition (Verlag Technik GmbH Berlin) discloses, generally, measures for ensuring electromagnetic compatibility.